The pressure vessel containing the core of water-cooled nuclear reactors in general, requires a continuous supply of water coolant to continuously carry heat away from the core. Even a momentary interruption in the coolant supply can result in serious damage to the core even though the core is thereafter promptly supplied with emergency core cooling water.
For example, a pressurized-water reactor normally comprises a pressure vessel containing a core barrel, in which the core is mounted, the core barrel being cylindrical and spaced from the inside of the cylindrical pressure vessel to form an annular space. At its top the barrel has a support flange which rests on a flange formed on the inside of the pressure vessel, the arrangement being such that the annular space is closed by the flanges from the balance of the area inside of the pressure vessel.
For core cooling, a coolant inlet nozzle opens through the pressure vessel wall into the upper portion of the above annular space, and a coolant outlet nozzle opens from the inside of the upper portion of the core barrel. The top and bottom of the core barrel are open. The reactor coolant system forms a pipe loop comprising a hot leg pipe welded to the pressure vessel's outlet nozzle and connecting with a steam generator, a cold pipe leg coming from the steam generator, via a coolant circulating pump, and back to the pressure vessel's inlet nozzle to which the pipe is welded. For a power reactor, the reactor coolant system normally includes a multiplicity of such coolant loops, the pressure vessel having coolant inlet and outlet nozzles for each loop. A pressurizer connected to the loop or loops keeps water coolant in the loops and in the pressure vessel under a pressure preventing the water from boiling, although operating at temperatures much above the boiling point of water at atmospheric pressure. The pressurized-water fed through the inlet nozzle, in the case of each loop, flows downwardly in the annular space between the core barrel and the pressure vessel inside, to the bottom of the vessel where the pressurized-water coolant rises up the core within the core barrel and flows outwardly through the coolant nozzle outlet, this circulation via the coolant loop being continuous.
In the event such a pressurized-water main coolant loop ruptures at any point, the loop pressure drops with a consequent drop in the pressure in the water on the pressure vessel, and the water in the pressure vessel rather immediately vaporizes and discharges in the direction of the leak, assuming the leak to be of major proportions. In such an event, the reactor protection system normally scrams the core while the emergency core cooling system forces emergency cooling water into the vessel, all this occurring as rapidly as possible. Even so, there is the chance that the vessel might become emptied of water coolant before such systems can become fully effective, and there is always the remote hypothetical possibility that one or another of the systems might not operate.
German Offenlegungsschrift No. 1,564,654 suggests that a check valve be positioned in one of the coolant pipes of such a pressurized-water reactor main coolant loop. Apparently the idea is that in the event of a break in the pipe line on the side of the check valve away from the pressure vessel, the check valve will be closed by the reversely flowing coolant discharging from the pressure vessel, thus preventing emptying of the pressure vessel, at least immediately.
There are objections to the above proposal. A check valve of the size required necessarily involves a large movable valve plate which because of the size involved, does not provide positive assurance of operation, and the fabrication cost of such a valve is very high. Provision of a safety redundancy by using two such valves in series, is an extremely expensive expedient. Furthermore, any break in the pipe line or connection between such a check valve and the pressure vessel nozzle, makes the check valve useless for its intended purpose.